1. Field of the Invention
This invention relates to a temperature measurement device and more particularly, to a device for the measurement of the temperature of a molten metal.
2. Background of the Invention
FIGS. 1 and 2 show the configuration of a conventional immersion sensor 10 for measuring the temperature of molten metals. The sensor 10 includes a thermocouple element 11 consisting of pairs of noble metal wires, typically platinum and alloys of platinum and rhodium. The thermocouple 11 is sheathed in a fused silica tube 12. The fused silica tube 12 is bent in the shape of a “U” such that a junction 13 of the Pt/Pt—Rd thermocouple is at the top of the bend. The wires of the thermocouple 11 exit the U-shaped silica tube 12 at open ends of the tube 12 opposite the bend in the tube 12. The wires of the thermocouple 11 are attached to copper alloy wires 14 of a type known to those skilled in the art. The assembly of thermocouple 11, the U-shaped tube 12 and the wires of the thermocouple 11 is potted in a heat resistant cement 15 filling the interior of a heat resistant ceramic body 16. This conventional construction, when coupled with suitable voltage measuring instrumentation, (not shown), is capable of accurately measuring the temperature of molten metal up to the temperature of melting of the thermocouple metals.
When sensors of this type are immersed in molten metal where the metal is flowing in a current, the fused silica tube 12, used to support the thermocouple element 11, can break or bend in the direction of the metal flow (see FIG. 3). Since fused silica is a glass, there exists a temperature where the fused silica begins to exhibit visco-elastic behavior. At temperatures lower than about 1000° C. the fused silica tube 12 tends to break as a response to the pressure of the flowing molten metal. In general there is no measurable viscous behavior below this temperature. Above 1000° C., the fused silica tube 12 deforms with the amount of permanent deformation depending upon the temperature, the amount of applied load and the purity of the fused silica. The problem of bending or breaking is more acute when the flow of molten metal is in a direction perpendicular to the plane of the fused silica “U” tube, as shown in FIG. 3.
In order to mitigate the bending or breaking of the fused silica tube, one approach suggested by the prior art is that of protecting the fused silica tube with a covering of higher temperature refractory material. Besides being more costly, the addition of higher temperature refractory materials on the fused silica tube introduces a mismatch of thermo-expansion between the fused silica tube, (very little expansion), and the added new material. When properly arranged as one skilled in the art would attempt, i.e., locating the faster expanding materials on the free surface of the U-shaped tube towards the molten metal in order to minimize the differences of thermo-expansion, a separation distance is inadvertently added between the temperature sensing element and the molten metal bath. This separation increases the thermo-response time of the sensor and therefore increases the exposure time of the sensor to the harsh environment resulting in a reduction of performance through failure of other components of the sensor.
The prior art also suggests substituting higher temperature materials for the fused silica tubes. Generally, the available higher temperature materials are not suitable due to commercial cost considerations. Alternatively, additional layers of materials may be added to the fused silica tube to reduce the deformation of the tube. Such an approach generally results in an unsuitable thermo-response time to a temperature change. Another approach of adding sleeves to the existing silica tube is not cost effective.